Mass flow controllers for gases sense the mass flow rate of a gas substantially independently of gas temperature or pressure, provide a measurement, and thus meter such flow, and adjust the mass flow rate, as desired, based on such sensing and metering. Forms of such controllers which operate on heat transfer principles have been widely adopted.
A common commercial form of a mass flow sensor for a gas incorporates a small diameter tube which has two coils of wire wound on the outside in close proximity to each other. The coils are formed from a metallic material having a resistance which is temperature-sensitive.
In a bridge-type electrical circuit, incorporated into the sensor, the coils can then be heated by an electrical current to provide equal resistances in the absence of flow of the gas and a balanced condition for the bridge-type circuit--e.g., a null output signal.
Then, with the gas flowing within the tube, within the relevant measuring range of the sensor, the temperature of the upstream coil is decreased by the cooling effect of the gas and the temperature of the downstream coil is increased by the heat from the upstream coil transmitted by the fluid. This difference in temperature is proportional to the number of molecules per unit time flowing through the tube. Therefore, based on the known variation of resistance of the coils with temperature, the output signal of the bridge circuit provides a measure, or a meter signal, of the gas mass flow.
Typically, the small diameter tube is in parallel with the primary fluid flow route through the controller. And this parallel path is set up using some form of partial fluid flow blockage setting up a pressure drop along the primary path.
With the flow measurement at a given level, and a desired flow setting at a different level, the difference can be eliminated, i.e., the mass flow rate can be controlled, through a valve in the primary fluid flow path.
Typically, in mass flow controllers, the primary fluid flow path is somewhat circuitous to allow for the mounting of a valve seat in a valve sleeve, which is opened or closed in a linear fashion. Thus, the member which is moved to increase or decrease the fluid flow through the valve is typically moved through a force applied along the direction of the fluid flow (the direction through the opening of the valve seat). Typically, the member acting against the valve seat is moved by a structure having a position which is controlled by a solenoid or piezo-electric mechanism, and is often attached to a metallic diaphragm. Such circuitous flow path and linear mechanisms tend to require custom-made conduit parts and larger and/or more forceful mechanisms than may otherwise be desirable. They also tend to be sensitive to back-pressure effects in the primary flow path which may complicate the mechanical operation of the valve.
Somewhat related to all this is a manufacturing requirement that can occur, relating to the mounting of a sleeve containing the valve seat in the primary fluid flow path. Specifically, there may be a requirement to avoid scratching along the primary fluid flow path in putting this sleeve in position. Where, for example, a press-fit operation is used, the inevitable scratching, then, should occur along wall surfaces which do not contact the fluid during fluid flow.
Concerning the need for some blockage of the primary fluid flow path in connection with providing a parallel path through the small diameter sensor tube, a variety of blockage elements have been used and attempted. This includes, for example, employing a primary fluid flow path having a taper, and a blockage structure having a taper which can be positioned, and then held in position through a spring-like mounting mechanism. Of concern in all this is some need to maintain rather smooth, typically laminar-type flow, in some vicinity of the location of the sensor-tube parallel path, and the capability to adjust the amount of blockage in order to adjust the ratio of flow through the sensor tube to flow through the primary path. This capability to adjust permits calibrating, within certain ranges, the flow rate which a given sensor signal represents. Such calibration flexibility is important in satisfying a variety of uses and individualized requirements.
In accordance with this, with regard to fluid flow blockage and adjustment, a generally circular, cylindrical flow path portion with a generally circular, cylindrical plug portion having wire-like structures along its length, as a blockage arrangement, is known. Then, the length of the cylindrical blockage portion which is disposed in the like-shaped flow path portion can be used as a way of controlling the degree of blockage.
In mass flow controllers of the general type noted, another area of effort and concern has been in interfacing the small-diameter sensor tube and the primary fluid flow path. This interfacing, of course, occurs in the vicinity of the blockage element in the primary fluid flow path. Such interfacing involves a small-diameter tube and the need for its sealed communication, at either end, with openings to the primary fluid flow path. Exemplary of the conventional responses to this requirement is the incorporation of a metallic mounting block disposed between the sensor housing and the metallic conduit block in this vicinity through which the primary fluid flow path is bored. Such a block, then, has generally circular, cylindrical openings formed in it through which the sensor tube passes, at its ends, just prior to the openings in the conduit block connecting to the primary fluid flow path. A pair of metallic pieces, then, are press-fit into these openings. Such pieces are bored with small-diameter openings, through the middle, for the small-diameter sensor tube end, either input or output. Such metallic pieces, which are generally circular, cylindrical in shape, can have, e.g., circular edges formed on the face toward the conduit block. Then, washer-like elements, having cooperating circular edges, to form a fluid-tight seal can be squeezed between the interface mounting block and the conduit block, about the sensor tube near the ends of such tube. The metallic interface pieces can have a built-up configuration along those same faces to provide a convenient form for welding the sensor pipe, at the ends of the pipe, to the pieces. The particularities of the interface structures, and the use of space by the interface mounting block, as examples, are noteworthy considerations.
Where, with respect to mass flow controllers, there has been a desire to use heat mechanisms, rather than piezo-electric or solenoid mechanisms, to control flow through the flow controller valve, the need for significant force-has typically been a requirement. In accordance with this, the use of electrical heating of a wire, to in turn heat a surrounding tube, has been the subject of past effort and development. The mass of the tube, in expanding with the application of heat or contracting with the removal of heat, can then be the source of the required amount of force, while also a source of slowness in the response time.
The present invention is directed to ease and efficiency of operation, as well as ease and efficiency of construction, in relation to a number of areas. These include the mechanics of valve operation, the directional aspects of the primary path fluid flow, the restriction of primary path fluid flow in relation to a sensor fluid path, the mounting of the sensor
housing and the related interface between the sensor fluid path and the primary fluid path, and the mounting of the valve seat in the primary fluid flow path.